The Klokken Gabbro--Syenite Complex, South Greenland: Cryptic Variation and Origin of Inversely Graded Layering
The Klokken stock has an outer sheath of vertically banded alkali gabbro which passes through syenogabbros into unlaminated syenites surrounding a central layered syenite core. The layered series comprises sheets of granular syenite interleaved with drusy laminated syenite which shows repeated inver...
| Published in: | Journal of Petrology |
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| Main Author: | |
| Format: | Text |
| Language: | English |
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Oxford University Press
1979
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| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/20/4/653 https://doi.org/10.1093/petrology/20.4.653 |
| Summary: | The Klokken stock has an outer sheath of vertically banded alkali gabbro which passes through syenogabbros into unlaminated syenites surrounding a central layered syenite core. The layered series comprises sheets of granular syenite interleaved with drusy laminated syenite which shows repeated inversely graded mineral layering in which normal leucosyenite becomes progressively more hedenbergite-rich upwards. Tops to layers are nearly feldspar-free, fayalite—magnetite horizons. Model cumulate textures are exhibited, and cross-bedding and normally graded channel structures are encountered. Granular layers often rest on ultramafic layers and load structures occur at the junctions. The series is cut by a sheet of biotite-syenodiorite and finally by quartzsyenite aplites. Plagioclase (An 57 in gabbros) is progressively mantled by ternary alkali feldspar and is absent only in the layered series in which alkali feldspars cluster around Ab 65 Or 35 . Pyroxene shows unbroken evolution with respect to Fe/Fe + Mg, from augite to ferrohedenbergite and sodie hedenbergite, with acmite in aplites, but Al and Ti decrease suddenly between syenogabbro and unlaminated syenite stages. Olivines show parallel evolution but there is a jump in Fe: Mg between gabbros and unlaminated syenites from Fo 52 to Fo 34 , with subsequent steady evolution to Fo 3 Te 5 Fa 92 in laminated syenites. Biotites show steady increase in Fe/Fe + Mg throughout the syenites, approaching pure annite, but the gabbro-syenogabbro series show a reversal, interpreted as indicating crystallization under conditions of increasing f o 2 . Ilmenomagnetites show systematically varying minor element ratios, with a sudden drop in MgAl 2 O 4 content between gabbros and syenites. Amphiboles (ferroedenites and sodic hastingsites) appear only in the most fractionated syenites in the layered series. The Klokken chamber was lined by gabbro with only limited in situ fractionation to syenogabbro. The termination of gabbro crystallization was marked by ingress of water into the ... |
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